1. Field of the Invention
The present invention relates to a novel compound, its use as an initiator in anionic polymerizations yielding norbornene-terminated homopolymers of block copolymers, and the further use of said norbornene-terminated polymers as macromonomers in the preparation of graft copolymers.
2. Description of Related Art
Anionic polymerization proceeds by attack on a vinyl monomer of a basic (nucleophilic) species resulting in the heterolytic splitting of the double bond to produce a carbon anion followed by propagation of this ion. The most common initiators used in such polymerization reactions are the alkyl and aryl derivatives of alkali metals, particularly lithium alkyls. Organolithium initiators are particularly preferred since they are readily prepared by reaction of the lithium metal with alkyl or aryl halides and are soluble in the hydrocarbon solvents used in their preparation as well as solvents used in solution polymerization reactions. N-butyl lithium and sec-butyl lithium are generally preferred initiators used for the anionic polymerization of vinyl and diolefin monomers including vinyl aromatic monomers, acrylic and methacrylic monomers and diolefin monomers such as butadiene or isoprene. A representative detail of organo-lithium initiators and their method of preparation appears in U.S. Pat. No. 3,890,408.
In a further development of this chemistry, organolithium initiators containing vinyl unsaturation have been used to initiate polymerization of anionically polymerizable monomers to produce vinyl terminated macromolecules which may be then used as a macromeric component in the preparation of copolymers by ionic or free radical polymerization techniques to produce graft copolymers containing the vinyl macromonomer which provides the pendant graft chains. For example, U.S. Pat. No. 3,235,626 to Waack, assigned to Dow Chemical Company, describes a method for preparing graft copolymers of controlled branch configuration. It is described that the graft copolymers are prepared by first preparing a prepolymer by reacting a vinyl metal compound with an olefinic monomer to obtain a vinyl terminated prepolymer. After protonation and catalyst removal, the prepolymer is dissolved in an inert solvent with a polymerization catalyst and is thereafter reacted with either a different polymer having a reactive vinyl group or a different vinyl monomer under free-radical conditions.
This art suffers from two major limitations: 1) Though the use of vinyl lithium guarantees that each polymer chain has one vinyl end group, it is recognized and documented in the literature, such as R. Waack et al., Polymer, Vol. 2, pp. 365-366 (1961) and R. Waack et al. J. Org. Chem., Vol. 32, pp. 3395-3399 (1967), that vinyl lithium is one of the slowest anionic polymerization initiators. This slow initiation rate when used to polymerize styrene produces a polymer having a broad molecular weight distribution (Mw/Mn greater than 2), as a consequence of the ratio of the overall rate of propagation of the styryl anion to that of the vinyl lithium initiation. As a result, graft copolymers prepared from these macromonomers cannot have a uniform side chain molecular weight. 2) It is well known in the art that substituted vinyl compounds do not generally polymerize to high conversions. Conversion tends to decrease as the length of the side chain increases. Conversions of 50%, high for most substituted vinyls, will mean that the resulting graft copolymers will contain 50% of unreacted macromonomer. For most applications this level of ungrafted polymer is unacceptable.
A different approach towards the preparation of macromonomers containing terminal unsaturated functional groups is also disclosed in the art.
Sumitomo Chemical's Japanese Kokai 50013483-A discloses olefin copolymers prepared by the Ziegler-catalyzed reaction of ethylene and/or propylene and polystyrene end-capped with norbornene. The preparation of a styrene-ethylene graft copolymer is described in an example, wherein the polystyrene macromonomer is formed by reacting living n-BuLi capped polystyrene with 5-bromomethyl-2-norbornene.
In addition, polystyrene macromonomers capped with a norbornene group have been prepared by coupling a polystyrene anion with 5-bromomethyl norbornene in a mixed solvent (Chemical Abstracts No. CA104 (26) 225321 w, 1986) and these functional polystyrenes have been further disclosed used as a comonomer in the Ziegler-Natta polymerization of graft copolymers comprising a polyethylene backbone containing grafted polystyrene side chains (Chemical Abstracts No. CA107(20) 176624y, 1987).
Functionalized macromolecules are also disclosed by R. Milkovich et al. in U.S. Pat. No. 3,989,768 as well as in R. Milkovich et al. J. Appl. Polym. Sci., Vol. 27, 1982, pg. 4773. This work describes anionic polymerization of a number of monomers with active initiators, thereby forming monodisperse living polymer chains. These living chains are then reacted with a wide-range of termination agents to introduce substantially end-functionalized macromonomers. This route clearly improves the resulting macromer polydispersity and allows for a broader range of end functionality, but it introduces an uncertainty into the "purity" of the end-functional groups. One can no longer be assured that each and every chain has one functional group. For example, the synthesis of norbornenylpolystyrene in accordance with the Milkovich journal article involves as step 1, the anionic polymerization of styrene in benzene using secondary-butyl lithium as initiator. This step, if done correctly, can be substantially free of termination. However in practice it is usually about 95% free of termination. Step 2 involves introducing ethylene oxide into the polymerization vessel to give the alkoxide. Once again this is about 95 % efficient. Step 3 involves the reaction of 5-norbornene-2-carbonyl chloride with the polystyrene alkoxide. This step is perhaps at best 95% efficient. Though each step results by any standards in excellent yields together they represent a polymer that is 0.95.times.0.95.times.0.95=86% end functional. Analytical techniques still have not reached the level of precision necessary to characterize this level of end-functionality of high molecular weight macromers. The most informative characterization comes from analysis of the graft copolymers produced using these macromers.
Synthesis of the graft copolymers using these macromers was presented in Milkovich U.S. Pat. No. 3,989,768 with very limited graft copolymer characterization information. A recent paper, B. Huang et al., J. of Polymer Science: Part A: Polymer Chemistry Edition, Vol. 24, 1986, pgs. 2853-2866 utilized the vinyl terminated macromer as described in U.S. Pat. No. 3,989,768 to prepare graft copolymers of ethylene and propylene. This work highlights two important points: First that double bond titrations can only give an approximation for end-group functionality and the best accuracy one can hope for is 20%. Second that the best conversions for vinyl terminated polystyrene macromonomers with a moderate molecular weight and useful feed compositions (10 to 30% on EP) is 40%.
In light of the above work, it is clearly highly desirable to devise a means for preparing macromonomers wherein the guaranteed functionality introduced in the initiation step is combined with a more active polymerization group. Also, in view of the utility of graft polymers of anionically polymerized macromonomers with alpha-olefin base polymers and particularly in view of the limitations and uncertainties in the prior art methods of preparing them, there exists an ongoing need for new and efficient means of preparation of graft polymers having essentially uniform molecular weight side chains. It is thus an object of this invention to provide novel compounds, novel macromolecules, and novel graft copolymers as well as novel means of preparation that allow for both rapid initiation of the anionically polymerized macromonomers and maximization of their functionalization for subsequent graft copolymerization.